In an MRI system or MR scanner, an examination object, usually a patient, is exposed to a uniform main magnetic field (B0 field) so that the magnetic moments of the nuclei within the examination object tend to rotate around the axis of the applied B0 field (Larmor precession) with a certain net magnetization of all nuclei parallel to the B0 field. The rate of precession is called Larmor frequency which is dependent on the specific physical characteristics of the involved nuclei, namely their gyromagnetic ratio, and the strength of the applied B0 field. The gyromagnetic ratio is the ratio between the magnetic moment and the spin of a nucleus.
By transmitting an RF excitation pulse (B1 field) which is orthogonal to the B0 field, generated by means of an RF transmit antenna, and matching the Larmor frequency of the nuclei of interest, the spins of the nuclei are excited and brought into phase, and a deflection of their net magnetization from the direction of the B0 field is obtained, so that a transversal component in relation to the longitudinal component of the net magnetization is generated.
After termination of the RF excitation pulse, the relaxation processes of the longitudinal and transversal components of the net magnetization begin, until the net magnetization has returned to its equilibrium state. MR relaxation signals which are emitted by the transversal relaxation process, are detected by means of an MR/RF receive antenna.
The received MR signals which are time-based amplitude signals, are Fourier transformed to frequency-based MR spectrum signals and processed for generating an MR image of the nuclei of interest within an examination object. In order to obtain a spatial selection of a slice or volume within the examination object and a spatial encoding of the received MR signals emanating from the slice or volume of interest, gradient magnetic fields are superimposed on the B0 field, having the same direction as the B0 field, but having gradients in the orthogonal x-, y- and z-directions. Due to the fact that the Larmor frequency is dependent on the strength of the magnetic field which is imposed on the nuclei, the Larmor frequency of the nuclei accordingly decreases along and with the decreasing gradient (and vice versa) of the total, superimposed B0 field, so that by appropriately tuning the frequency of the transmitted RF excitation pulse (and by accordingly tuning the resonance frequency of the MR/RF receive antenna), and by accordingly controlling the gradient fields, a selection of nuclei within a slice at a certain location along each gradient in the x-, y- and z-direction, and by this, in total, within a certain voxel of the object can be obtained.
The above RF (transmit and/or receive) antennas are known both in the form of so-called MR body coils (also called whole body coils) which are fixedly mounted within an examination space of an MRI system for imaging a whole examination object, and as so-called MR surface coils which are directly arranged on a local zone or area to be examined and which are constructed e.g. in the form of flexible pads or sleeves or cages (head coil or birdcage coil).
As to the shape of the examination space, two types of MRI systems or MR scanners can be distinguished. The first one is the so-called open MRI system (vertical system) which comprises an examination zone, which is located between the ends of a vertical C-arm arrangement. The second one is an MRI system, also called axial MRI system, which comprises a horizontally extending tubular or cylindrical examination space.
U.S. Pat. No. 4,799,016 discloses a dual frequency NMR surface coil having a first cylindrical cage coil which is caused to resonate at a lower RF frequency, and a second cylindrical cage coil which is caused to resonate at a higher RF frequency, wherein both cage coils surround a common region of interest along a central axis, and the lower RF frequency is the Larmor frequency of phosphorus and the higher RF frequency is the Larmor frequency of hydrogen. By this, two separate NMR signals from the two different species of nuclei can be received and displayed for the same examination object.